1. Field of the Invention
The present invention relates generally to xerographic or electrophotographic copiers, printers, and the like, and particularly to methods and apparatus for selective discharge of a charged photoconductor to form a developable latent image for transfer to a copy sheet which is ultimately processed to produce a visible copy of the image. The present invention makes it possible to obtain the economical advantages of two-cycle xerographic processing with the operating speed of a single-cycle electrophotographic process.
2. Description of the Prior Art
Contemporary electrophotographic products are broadly characterized in general as either single-cycle machines or two-cycle machines. A typical single-cycle machine charges a photoconductor belt or drum, selectively discharges areas of that photoconductor to conform to a desired image, develops the image, as by application of toner, transfers the image to a copy sheet, discharges the photoconductor, and subsequently cleans the photoconductor with a cleaning station separate from the developer unit. Single-cycle machines operate over a wide range of operating speeds.
Unfortunately, the single-cycle machines suffer from the disadvantage of additional cost associated with single-function elements, such as a separate cleaning station. In addition, the separate cleaning station has also contributed a significant cost factor for servicing and maintenance of that equipment. For instance, cleaning brushes, filters, toner return augers, and the like, require servicing and changing or replacement at relatively frequent intervals, and thus place a significant operating cost on the machine owner.
Two-cycle machines use dual functions for each of several components associated with the electrophotographic processing. A prime example is shown in U.S. Pat. No. 3,647,293, entitled "Copying System Featuring Combined Developing/Cleaning Station Alternately Activated" by Carl A. Queener filed December 1, 1970, wherein shared function components are disclosed including a combined developer/cleaner alternately operable to provide those functions during separate cycles associated with the photoconductor.
While two-cycle machines do not require conventional xerographic cleaning stations and thus avoid the expenses associated therewith, a price is paid in throughput in that a separate cleaning cycle is executed each time the page printed or copied is changed. In a machine, such as a copier/duplicator using a recirculating document feeder or a laser or light-emitting diode xerographic printer that prints collated sets, the page printed or copied is changed every time, thus cutting the throughput in half for a given xerographic process speed or alternately doubling the process speed to maintain a given throughput.
Some prior art devices include interior lamps to illuminate the photoconductor belt and to assist in the discharge of the photoconductor in aid of cleaning. Such a system is shown in U.S. Pat. No. 4,372,669 by J. Fantuzzo et al. filed June 29, 1981, which utilizes a cleaning augmentation lamp within a photoconductor belt in a two-cycle machine environment. U.S. Pat. No. 4,322,734 by Y. Ebi et al. filed November 8, 1979, shows a similar arrangement wherein an internally located lamp illuminates a photoconductor belt, which has a transparent interior layer, with an intermediate photoconductor layer. The Ebi et al. device controls the intensity of the lamp for the purpose of maintaining relatively uniform copy density associated with an electrode discharge imaging assembly.
A photoelectrophoretic migration imaging system using an internally located imaging means, such as a laser which illuminates a transparent drum, is shown in U.S. Pat. No. 4,357,096 by C. L. Fetterman filed March 6, 1981. The Fetterman device sprays electrophotosensitive pigment onto the transparent drum exterior. At the imaging zone, illuminated portions from the interior source are caused to migrate to an appropriate one of either the transparent drum or a tangentially engaging drum with the portions so migrating originating from the pigment supply spray. The Fetterman device is essentially a single-cycle configuration in that a separate pigment cleaning station is employed.
None of the known prior art obtains the advantages of avoiding use of a separate cleaning station associated with a two-cycle machine while enjoying the operating speed of a single-cycle machine. This result is obtained by the present invention.